1. Field of the Invention
The present invention relates to a vertical amplifier apparatus with a beam-finding function for an oscilloscope.
2. Description of the Prior Art
In general, there is a case where an input signal for an oscilloscope, which includes a large DC offset value for the vertical input range, disappears beyond the display range. In this case, the observer utilized the beam-finding function to check the input signal for a plus or minus DC offset.
FIG. 1 is a block diagram showing schematic arrangement of a two quadrant oscilloscope. Each channel includes an attenuator 31, a position controller 32, a vertical amplifier 33. As a common stage, there are provided a multiplexer 34, a final stage amplifier 35 and a CRT 36 (Cathod Ray Tube). Here, the vertical amplifier 33 includes an input feedback amplifying state 33a and an output feedback amplifying stage 33b, which will be described later. the gain of the vertical amplifier can be adjusted by varying the volume of a variable resister 33c included in the amplifier 33. In the conventional oscilloscope, a beam-finder circuit 37 is connected to the final stage amplifier 35. The operation of this circuit will be described.
Transistors Q41 and Q42, and resistors R41 and R42 constitute a differential feedback amplifier of the final state amplifier 35, and a transistor Q43 and resistors R44 and R45 constitute the beam-finder circuit 37. The base of the transistor Q43 of the beamfinder circuit 37 normally controlled so as to close the transistor Q43, and therefore, the mixed emitter current of the transistors Q41 and Q42 flows through the resistors R43 and R44 connected in parallel. In the beam-finding operation, the emitter current flows only through the resistor 43 because the transistor Q43 is opened, and therefore, the gain decreases with the emitter current which is less than that of normal operation. Supposing the resistance value of the resistor R3 is much smaller than that of the resistor R44, the difference of the gain between in the normal operation and in the beam-finding operation will increase. In this way, the gain of the final stage amplifier can be decreased. As a result, the input signal wave, which is beyond the display range of CTR 36, is reduced to be displayed within the range. Furthermore, the Z axis (brightness) is controlled at the same time. The reason why the Z axis if controlled is that the rough condition of the input signal in the beam-finding operation ca ben checked by increasing the brightness even if no trigger is applied in the trigger sweep mode.
In such a conventional circuit, the beam-finder circuit 37 is connected to the amplifier 35 which is connected to the output of the multiplexer 34. When the beam-finder circuit 37 is actuated while a plurality of input signals are applied, gains of both vertical axis and horizontal axis are uniformly reduced for all the input signals by controlling the amplification of the final state amplifiers by using the beam-finder circuit. For this reason, the beam-finding function cannot be independently applied to each channel. In order to solve this problem in the conventional circuit, the vertical amplifier 33 of each channel must be provided with a resistor for reducing the amplification of the vertical amplifier 33. This solution, however, presents such a new problem that the component number of the beam-finder circuit increases and the manufacturing cost is raised.